Assume that a plane wave in air with an electric field $\overrightarrow{E}= 10\cos ( \omega t-3x-\sqrt{3}z )\hat{a_{y}}V/m$ is incident on a non-magnetic dielectric slab of relative permittivity $3$ which covers the region $z > 0$. The angle of transmission in the dielectric slab is _________ degrees.